Optical inspections methods may be used to determine whether a glazing meets various safety standards. Typically, optical inspection methods are used to determine whether a glazing meets optical standards. However, optical inspection methods may also be used to help determine whether a glazing meets impact resistance standards.
Most of windowpanes for automotive and some glass panel for architectural use are made of tempered glass, which has held compressive stress applied to surfaces thereof to improve resistance to tensile stress. Tempering puts the outer surfaces into compression and the inner surfaces into tension. Such stresses cause the glass, when broken, to crumble into small granular chunks instead of splintering into jagged shards as plate glass create. The granular chunks are less likely to cause injury. As a result of its safety and strength, tempered glass is used in a variety of demanding applications, including windows, shower doors, architectural glass doors and tables, refrigerator trays, as a component of bulletproof glass, for diving masks, and various types of plates and cookware.
In the production of such a kind of glass, a fragmentation test is carried out as a quality assurance test.
Particularly, respectively for automotive use and architectural use, the fragmentation test for tempered glass are prescribed by ECE R43 and EN12150 standards which are the relevant safety standard in Europe.
The fragmentation test for tempered glass is a test wherein test glass is fragmented by applying an impact shock to a certain portion thereof with a punch, the number of fragments in a region with coarsest fragment included and in a region with the finest fragment included are calculated, and the area of the greatest fragment and the length of the longest fragment in that region are measured to see whether test glass meets desired specifications, or not.
For the calculation of the fragments or another purpose, a measuring method has been adopted wherein the image of fragmented test glass is exposed on photosensitive paper to obtain an image as a blueprint (herein below, referred to as a blueprinted image), and the measurement is conducted utilizing the blueprinted image. In the measuring method, all operations including the setting of selected regions and the calculation of the number of the fragments have been manually carried out based on the blueprinted image. The conventional measuring method has required considerable labor in the calculating operation for counting the number of the fragments and another operation.
As an example of systems to improve the reproducibility of the fragmentation test, “Automated calculating system for the ECE fragmentation test” of Ford Motor Co. (GLASS PROCESSING DAYS, 13-15 Sep. 1997) discloses a technique of the calculating operation for counting the number of the fragments and another operation in the blueprinted image are automatically carried out by a computer. According to this automated calculating technique, there is no need for an operator to manually count the number of the fragments, reducing the number of the steps required for the calculating operation.
Since the measurement of the number of the fragments and another operation have been manually made by an operator in the conventional manual measuring method for the fragmentation test the conventional manual measuring method has created some difficulties in that the calculating operation in the fragmentation test requires many steps and much labor cost.
An alternative method of imaging and processing the test data is disclosed in U.S. Pat. No. 6,766,046. A light source is positioned above a ply of glass supported on a paper screen held on a transparent guide sheet. A camera having a line sensor is positioned underneath the screen, to detect the image of the fragments projected onto the screen by the light source. Either the glass or the camera may be moved to ensure that the entire area of the ply of glass is scanned. The light source may be a point source, combined with a condensing lens, or an array of light sources.
The automation of the data collection and image processing reduces the time necessary to determine whether a ply of glass has passed or failed the fragmentation test. However, the use of the screen (where the camera records the screen image rather than a direct image from the glass) causes difficulties when glass having a low light transmission is tested. In U.S. Pat. No. 6,766,046, low light transmission is overcome by employing a photosensor to determine the light transmission of the glass so that the exposure time needed by the camera to record the image on the screen may be adjusted accordingly. However, for low light transmission glass, this increases the time required for data acquisition. For an automated system to be viable in a production situation, the image capture ideally needs to be completed within a three minutes time for a window.
There is therefore a need for an optical inspection system allows testing and measurement for the fragmentation test under, for example, ECE R43 for automotive glazing or EN12150 for architectural glazing to be carried out within a short time-frame, for both high and low light transmission glasses.